The present invention relates to a method of and a system for controlling a photovoltaic power system comprising a solar battery in such a way as to maximize the output power of the solar battery.
Importance of solar batteries is increasing from the viewpoint of effective utilization of energy and it is expected that solar batteries will find wide applications such as domestic use, linking with electric power systems. Solar batteries convert solar energy into electrical energy. For efficient utilization, it is desirable that the output power of the solar batteries be kept maximized regardless of variation in the atmospheric conditions.
FIG. 1 shows an example of photovoltaic power system comprising a solar battery 1 and a voltage-type inverter. The output of the inverter 2 is connected through coupling reactors 3 to AC power source 4 which may for example be a power distribution network. The output voltage of the solar battery as detected by a voltage detector 5 and a voltage reference from a voltage reference setter 6 are compared at a subtractor 7, and the deviation as determined by the subtractor 7 is applied to an inverter control circuit 8 which controls the inverter 2 in such a manner as to make the deviation smaller (closer to zero) or to keep the deviation at zero. When the detected voltage is larger than the voltage reference the inverter 2 is so controlled as to increase its output power. When, on the contrary, the detected voltage is smaller than the voltage reference the inverter 2 is controlled as to reduce its output power. In this way, the deviation of the detected voltage from the voltage reference is made smaller.
In a prior art system, the voltage reference is determined to maximize the output power in accordance with a characteristic of the solar battery. For instance it is assumed that, for a given insolation (light intensity), the voltage maximizing the output power is constant or is linearly related to the current, and the voltage reference is determined on such an assumption. Where the voltage is assumed to be linearly related to the current, a detected current value may be inputted to the voltage reference setter 6. In many of the solar batteries, the maximum output power can be nearly perfectly tracked.
In another conventional method, an additional solar battery for monitoring the insolation is used to optimize the voltage reference for the detected insolation. The monitoring solar battery may be opened or short-circuited and the open voltage or the short-circuit current may be used to determine the voltage reference.
In the various methods described, it is essential that the characteristic of the solar battery is known, at least to some extent. Moreover, the maximum output power may not be accurately tracked. This is because the voltage and current of a solar battery depend not only on the insolation but also on temperature. Furthermore, when a solar battery is exchanged, the voltage reference may have to be changed because of a difference between their characteristics.